1. Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery, a method for preparing a negative active material for a rechargeable lithium battery, and a rechargeable lithium battery. More particularly, the present invention relates to a negative active material for a rechargeable lithium battery that shows high capacity due to high energy density per unit volume, high discharge potential, long cycle-life, and excellent safety, a method for preparing a negative active material for a rechargeable lithium battery, and rechargeable lithium battery.
2. Description of the Related Art
Various carbon-based materials, e.g., artificial and natural graphite, hard carbon, etc., may be used as a negative active material of a rechargeable lithium battery. These materials can intercalate and deintercalate lithium ions. Graphite may increase discharge voltages and energy density for a battery due to its low discharge potential, e.g., about 0.2V, compared to lithium. A battery using graphite as a negative active material may have a high average discharge potential, e.g., about 3.6V, and an excellent energy density. Graphite is most comprehensively used among the aforementioned carbon-based materials since graphite guarantees better battery life cycle due to its outstanding reversibility.
However, graphite has a low density (its theoretical density is 2.2 g/cc). Consequently, graphite has a low capacity in terms of energy density per unit volume, and, thus, has a low charge and discharge capacity when used as a negative active material. Further, the use of graphite may present potential risks, e.g., explosion or combustion, when a battery is misused, overcharged, etc., since graphite may react to an organic electrolyte at a high discharge voltage.
In order to solve these problems, a great deal of research on an oxide negative electrode has recently been performed. One material investigated has been amorphous tin oxide, which has a high capacity per weight (800 mAh/g). However, use of amorphous tin oxide has resulted in critical defects, e.g., a high initial irreversible capacity of up to 50%. In other words, 2nd cycle discharge capacity may be decreased up to less than one half of an initial discharge capacity. Additionally, amorphous tin oxide has a discharge potential of more than 0.5V, and its charge and discharge profile has a broad voltage variation. Consequently, tin oxide may not be suitable for use in a battery. Further, some of the tin oxide tends to be reduced into tin metal during the charge or discharge reaction, which further reduces its acceptability for use in a battery.
Other materials for use as a negative active material, e.g., LiaMgbVOc (0.05≦a≦3, 0.12≦b≦2, 2≦2c-a-2b≦5) and Li1.1V0.9O2, have been proposed. However, these do not sufficiently resolve all of the above problems.